Dissimilar diagenetic histories of Smackover Sandstones, Gulf of Mexico, United States of America

Diagenesis has had a profound effect on Late Jurassic Smackover sandstones. Petrographic and geochemical analyses were carried out on sandstones from over 90 oil wells spanning the Gulf of Mexico from southern Texas to western Alabama. Variations in detrital mineralogy are attributable to both provenance and pore fluid evolution whereas the history of cementation was controlled primarily by depositional environment and pore fluid evolution.;Smackover sandstones are very-fine to medium grained and are mineralogically mature to sub-mature. These sandstones have undergone extensive framework modification which resulted in the dissolution or replacement of 50 to 80 percent of the original feldspar. Restored framework composition, rock fragments, and quartz types of sandstones from the South Texas and Louisiana basins indicate that these detrital components were derived primarily from the interior zone and the fold and thrust belt of the Ouachita orogen respectively. Sandstones from East Texas and the Mississippi basin were derived primarily from the interior of the North American Craton.;Sandstones from the South Texas, East Texas and Mississippi salt basins have similar diagenetic histories. The paragenetic sequence for these rocks is: adularia, calcite, dolomite, quartz, calcite, and anhydrite. The first four cements precipitated at shallow burial depths and low temperatures. The silicate minerals derived their components from dissolving feldspars and the carbonates derived their components from sea water. Late anhydrite cements were derived from dissolving bedded anhydrite above or below the Smackover.;The paragenetic sequence for Louisiana basin sandstones is: quartz, ferroan dolomite, calcite, and anhydrite. The quartz formed at low temperatures from the components of dissolving feldspars. The components for the dolomite were derived later in the burial history from an unknown Mg-rich source and were carried by fluids that had undergone significant water-rock interaction. The anhydrite formed in a similar fashion to the other three basins. The lack of early dolomite cement in Louisiana basin sandstones was the result of a deep water depositional history.;The prevalence of low-temperature early silicate cements in Smackover sandstones suggests that early dissolution of feldspars might play a more important role in sandstone diagenesis than has been previously recognized.